Oxidative desulfurization of fuel oil

ABSTRACT

A method for purifying a sulfur-containing fuel oil comprising (a) contacting the fuel oil with a supported exogenous binary catalyst and oxygen at a temperature in a range of from about 25° C. to about 150° C., and at a pressure in a range of from about 1 atmosphere to about 150 atmospheres to provide a first oxidized mixture; and (b) separating at least one oxidized sulfur compound from the first oxidized mixture to a provide a purified fuel oil. In one embodiment, the sulfur-containing fuel oil is deasphalted prior to contacting with the supported exogenous binary catalyst and oxygen.

BACKGROUND

The invention includes embodiments that generally relate to a method for purifying sulfur-containing fuel oil using air and a catalyst.

Raw/fossil fuels, such as fuel oil including a crude oil and oil distillates and refinery products like gasoline, kerosene, diesel fuel, naphtha, heavy fuel oil, natural gas, liquefied natural gas and liquefied petroleum gas, and like hydrocarbons, are useful for a number of different processes, particularly as a fuel source, and most particularly for use in a power plant. Virtually all of these fuels contain relatively high levels of naturally occurring, organic sulfur compounds, such as, but not limited to, sulfides, mercaptans and thiophenes. Hydrogen generated in the presence of such sulfur compounds has a poisoning effect on catalysts used in many chemical processes, particularly catalysts used in fuel cell processes, resulting in shortening the life expectancy of the catalysts. When present in a feed stream in a fuel cell process, sulfur compounds may also poison the fuel cell stack itself. Because of the relatively high levels of sulfur compounds that may be present in many crude fuel feed streams, it is necessary that these feed streams be desulfurized.

Furthermore, desulfurization of fuels has become an important problem due to the upcoming regulatory requirements that require a reduction in current sulfur emissions. Two major tasks in the sulfur removal from fuel include (i) the deep desulfurization of diesel fuel (reducing S content from ˜500 parts per million to below 15 parts per million) and, (ii) sulfur removal from crude and heavy fuel oils used for energy production (reducing S content from 3-4 percent to less than 0.5 percent). Conventional hydrodesulfurization (HDS) method using hydrogen have not only been insufficient to effect the deep desulfurization of diesel fuels but are also relatively expensive for the direct sulfur removal from a crude and heavy fuel oils due to high cost of hydrogen and the use of high temperature and pressure. Alternatively oxidative desulfurization (ODS) methods using oxidants like hydrogen peroxide, molecular oxygen or ozone, require somewhat less demanding operating conditions when compared to the operating conditions employed in HDS methods. Further, where oxygen may be used as the stoichiometric oxidant, ODS methods may be cost competitive with HDS methods.

Thus, there exists a need for efficient and cost effective ODS methods for sulfur removal from fuel, to provide desulfurized fuels that meet modern engineering and regulatory standards.

BRIEF DESCRIPTION

In one embodiment, the present invention provides a method for purifying a sulfur-containing fuel oil comprising (a) contacting the fuel oil with a supported exogenous binary catalyst and oxygen at a temperature in a range of from about 25° C. to about 150° C., and at a pressure in a range of from about 1 atmosphere to about 150 atmospheres to provide a first oxidized mixture; and (b) separating at least one oxidized sulfur compound from the first oxidized mixture to a provide a purified fuel oil.

In another embodiment, the present invention provides a method for purifying a sulfur-containing fuel oil comprising (a) contacting the sulfur-containing fuel oil with a hydrocarbon diluent, an alumina supported exogenous binary catalyst, and oxygen at a temperature in a range of from about 50° C. to about 120° C., and at a pressure in a range of from about 1 atmosphere to about 50 atmospheres to provide a first oxidized mixture; (b) separating at least one oxidized sulfur compound from the first oxidized mixture; and (c) recovering the diluent to provide a purified fuel oil.

In yet another embodiment, the present invention provides a method for purifying a sulfur-containing fuel oil comprising (a) contacting a sulfur-containing fuel oil comprising benzothiophene, dibenzothiophene, alkyl substituted benzothiophenes, and alkyl substituted dibenzothiophenes with petroleum-ether, a supported exogenous binary catalyst, and oxygen at a temperature in a range of from about 50° C. to about 120° C., and at a pressure in a range of from about 1 atmosphere to about 50 atmospheres to provide a first oxidized mixture comprising sulfoxides and sulfones of benzothiophene, dibenzothiophene, alkyl substituted benzothiophenes, and alkyl substituted dibenzothiophenes; (b) separating at least one oxidized sulfur compound from the first oxidized mixture; and (c) recovering petroleum-ether to provide a purified fuel oil.

These and other features, aspects, and advantages of the present invention may be understood more readily by reference to the following detailed description.

DETAILED DESCRIPTION

In the following specification and the claims, which follow, reference will be made to a number of terms, which shall be defined to have the following meanings.

The singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.

“Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.

Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.

In one embodiment, the present invention provides a method for purifying a sulfur-containing fuel oil comprising (a) contacting the fuel oil with a supported exogenous binary catalyst and oxygen at a temperature in a range of from about 25° C. to about 150° C., and at a pressure in a range of from about 1 atmosphere to about 150 atmospheres to provide a first oxidized mixture; and (b) separating at least one oxidized sulfur compound from the first oxidized mixture to a provide a purified fuel oil.

In one embodiment, the sulfur-containing fuel oil is a crude oil, for example Saudi sweet crude oil, West Texas Intermediate crude oil, Dubai crude oil, and Brent crude oil. In an alternate embodiment, the sulfur-containing fuel oil is a crude oil, which has been subjected to asphaltene removal. In one embodiment, the sulfur-containing fuel oil is a distillate or other refinery products of a crude oil like gasoline, kerosene, diesel fuel, naphtha, heavy fuel oil, natural gas, liquefied natural gas and liquefied petroleum gas. In one embodiment, the sulfur-containing fuel oil comprises dibenzothiophene, benzothiophene, alkyl substituted dibenzothiophenes, and alkyl substituted benzothiophenes.

In one embodiment, the sulfur-containing fuel oil comprises less than 5 weight percent sulfur based on the weight of sulfur-containing fuel oil. In another embodiment, the sulfur-containing fuel oil comprises less than 3 weight percent sulfur based on the weight of sulfur-containing fuel oil. In another embodiment, the sulfur-containing fuel oil comprises less than 2 weight percent sulfur based on the weight of sulfur-containing fuel oil.

In one embodiment, the supported exogenous binary catalyst comprises a solid support comprising a metal oxide. As used herein the phrase “supported exogenous binary catalyst” means a “supported external binary catalyst” that is combined in a first reaction mixture with a sulfur-containing fuel oil. In one embodiment, the metal oxide solid support may be selected from the group consisting of alumina, silica, magnesia, titania, ceria, and combinations of at least two of the foregoing. In one embodiment, the solid support is alumina.

In one embodiment, the binary catalyst comprises a first component, a catalyst and a second component, a promoter. In one embodiment the binary catalyst comprises a first component selected from the group consisting of oxides and salts of vanadium, manganese and copper, and combinations thereof, and a second component selected from the group consisting of oxides and salts of cerium, iron, titanium, manganese, cobalt, nickel, copper and combinations thereof. In another embodiment, the first component comprises an oxide or a salt of vanadium. In yet another embodiment, the first component comprises an oxide or a salt of manganese. In another embodiment, the second component comprises an oxide or a salt of cobalt. In yet another embodiment, the second component comprises an oxide or a salt of copper.

In one embodiment, the binary catalyst comprises an active metal component which is present in an amount corresponding to from about 1 weight percent to about 10 weight percent based on the weight of the support. In another embodiment, the binary catalyst comprises an active metal component which is present in an amount corresponding to from about 2 weight percent to about 8 weight percent based on the weight of the support. In yet another embodiment, the binary catalyst comprises an active metal component which is present in an amount corresponding to from about 4 weight percent to about 6 weight percent based on the weight of the support.

In one embodiment, when the amount of active metal component is in a range of from about 1 weight percent to about 10 weight percent based on the support, the amount of binary catalyst on the metal oxide support employed in the oxidation reaction is in a range of from about 0.25 weight percent to about 10 weight percent based on the amount of the sulfur-containing fuel oil. In another embodiment, the amount of binary catalyst on the metal oxide support employed in the oxidation reaction is in a range of from about 0.5 weight percent to about 8 weight percent based on the amount of the sulfur-containing fuel oil. In yet another embodiment, the amount of binary catalyst on the metal oxide support employed in the oxidation reaction is in a range of from about 1 weight percent to about 5 weight percent based on the amount of the sulfur-containing fuel oil. One skilled in the art can easily determine the amount of metal catalyst on a metal oxide support required for the oxidation reaction based on the amount active metal content present in the metal oxide support and the amount of sulfur-containing fuel oil being purified.

In one embodiment, the amount of second component is in a range of from about 4 weight percent to about 50 weight percent based on the amount of the first component employed. In another embodiment, the amount of second component is in a range of from about 5 weight percent to about 12 weight percent based on the amount of the first component employed. In yet another embodiment, the amount of second component is in a range of from about 6 weight percent to about 10 weight percent based on the amount of the first component employed.

In one embodiment, the at least one oxidized sulfur compound may be separated from the first oxidized mixture using a solid-liquid extraction process, for example an adsorption process, to provide the purified fuel oil. In one embodiment, the at least one oxidized sulfur compound may be separated from the first oxidized mixture using a liquid-liquid extraction process, to provide the purified fuel oil. One skilled in the art can easily determine the process and the conditions required to achieve satisfactory separation.

In one embodiment, the method for purifying the sulfur-containing fuel oil further comprises a step of recovering the supported binary catalyst. In one embodiment, the supported binary catalyst is recovered from the first oxidized mixture by filtration or centrifuging/decantation, using methods known to one skilled in the art.

In one embodiment, the first oxidized mixture is contacted with a porous silica adsorbent material, wherein the adsorbent material is characterized by a Brunauer-Emmett-Teller (BET) surface area value (total) of at least about 15 m²/g; and a Barrett-Joyner-Halenda (BJH) pore volume (total) of at least about 0.5 cc/g. Such porous adsorbent materials and their use are described in copending U.S. patent application Ser. No. 11/934298 filed Nov. 2, 2007 which is incorporated herein by reference in its entirety. In instances wherein the sulfur-containing fuel oil comprises other metallic impurities such as vanadium compounds, such contact results in removal of these other metallic impurities or their oxidation products from the first oxidized mixture.

In one embodiment, the pressure at which the oxidation (also referred to as contacting the fuel oil with a supported exogenous binary catalyst and oxygen at a temperature in a range of from about 25° C. to about 150° C., and at a pressure in a range of from about 1 atmosphere to about 150 atmospheres to provide a first oxidized mixture) is carried out is in range of from about 1 atmosphere to about 150 atmospheres. In another embodiment, the pressure at which the oxidation is carried out is in range of from about 5 atmospheres to about 45 atmospheres. In yet another embodiment, the pressure at which the oxidation is carried out is in range of from about 10 atmospheres to about 40 atmospheres.

In one embodiment, the temperature at which the oxidation is carried out is in a range from about 25° C. to about 150° C. In another embodiment, the temperature at which the oxidation is carried out is in a range from about 50° C. to about 120° C. In yet another embodiment, the temperature at which the oxidation is carried out is in a range from about 60° C. to about 90° C.

In one embodiment, the oxygen required for contacting the sulfur-containing fuel oil is provided as a mixture with an inert gas. Non-limiting suitable examples of gases suitably inert to the conditions employed include nitrogen and argon. In one embodiment, the oxygen is provided as air.

In another embodiment, the sulfur-containing fuel oil is deasphalted prior to contacting the sulfur-containing fuel oil with the binary catalyst and oxygen. Deasphaltation of the sulfur-containing fuel oil may be carried out by methods known to one skilled in the art. Typically, deasphaltation is carried out by contacting the sulfur-containing fuel oil with an inert diluent and filtering or centrifuging the resultant mixture to separate the fuel oil from the insoluble asphaltenes to provide a deasphalted fuel oil. In one embodiment, the inert diluent is selected from the group consisting of liquid saturated hydrocarbons, liquid cyclic hydrocarbons, and mixtures of at least two of the foregoing inert diluents. Suitable non-limiting examples of liquid cyclic hydrocarbons include cyclohexane, cycloheptane, and decalin. Suitable non-limiting examples of liquid saturated hydrocarbons include propane, butane, and petroleum ether. In one embodiment, the method for purifying the sulfur-containing fuel oil further comprises a step of recovering the inert diluent. In one embodiment, the inert diluent is recovered from the first oxidized mixture by distillation, using methods known to one skilled in the art.

In another embodiment, the present invention provides a method for purifying a sulfur-containing fuel oil comprising (a) contacting the sulfur-containing fuel oil with a hydrocarbon diluent, an alumina supported exogenous binary catalyst, and oxygen at a temperature in a range of from about 50° C. to about 120° C., and at a pressure in a range of from about 1 atmosphere to about 150 atmospheres to provide a first oxidized mixture; (b) separating at least one oxidized sulfur compound from the first oxidized mixture; and (c) recovering the diluent to provide a purified fuel oil.

In yet another embodiment, the present invention provides a method for purifying a sulfur-containing fuel oil comprising (a) contacting a sulfur-containing fuel oil comprising benzothiophene, dibenzothiophene, alkyl substituted benzothiophenes, and alkyl substituted dibenzothiophenes with petroleum-ether, a supported exogenous binary catalyst, and oxygen at a temperature in a range of from about 50° C. to about 120° C., and at a pressure in a range of from about 1 atmosphere to about 50 atmospheres to provide a first oxidized mixture comprising sulfoxides and sulfones of benzothiophene, dibenzothiophene, alkyl substituted benzothiophenes, and alkyl substituted dibenzothiophenes; (b) separating at least one oxidized sulfur compound from the first oxidized mixture; and (c) recovering petroleum-ether to provide a purified fuel oil.

The following examples are intended only to illustrate methods and embodiments in accordance with the invention, and as such should not be construed as imposing limitations upon the claims.

EXAMPLES

Reagents and certain of the catalysts employed herein were obtained from Aldrich Chemical Company.

Catalysts were prepared using an INCIPIENT WETNESS protocol known in the art incorporating a gamma-alumina support and aqueous solutions of nitrate or acetate salts of a transition metal (In one instance vanadyl acetylacetonate was used for the preparation of vanadium pentoxide). After impregnation of the catalyst in the alumina support, the samples were dried at 120° C. and calcined in air at about 550° C. for about 5 hours. The temperature was increased from 120° C. to about 550° C. at a rate of 5° C./min. The amount of active metal component used was about 5 weight percent based on the weight of the alumina support.

Examples 1 to 9 and Comparative Examples CE-1 to CE-2 Effect Of Oxidative Desulfurization on a Sulfur-Containing Fuel Oil Model Mixture

A sulfur-containing model mixture was prepared from tetralin and dioctylsulfide (DOS), benzothiophene (BT), and dibenzothiophene (DBT) wherein the sulfur-containing compounds were present in a 2:2:3 weight ratio. The model mixture was shown to comprise about 3 weight percent sulfur, when tested using a Varian Saturn 2000 GCMS. 7 milliliters (ml) of the mixture and 50 milligram (mg) supported exogenous binary catalyst were placed in each of 6 four-dram vials equipped with magnetic cross-like stirbars. The vials were placed in an aluminum heating block. The block with the vials was placed in a one-gallon autoclave equipped with a magnetic stirrer. The autoclave was maintained under an air pressure of 2000 pounds per square inch at a temperature of about 150° C. for a period of about 6 hours. The autoclave was then depressurized and the oxidized samples analyzed using the Varian Saturn 2000 GCMS. The results are presented in Table 1 below.

TABLE 1 Conversion of sulfur compounds in a fuel oil model mixture under air oxidation in the presence of exogenous binary catalysts supported on alumina. Catalyst Conversion Percentage Example 1^(st) component 2^(nd) component BT DBT DOS 1 MnO₂ Co₃O₄ 23.0 42.6 100 2 MnO₂ CuO 31.0 41.4 100 3 MnO₂ Co₃O₄ 25.3 44.4 100 4 MnO₂ Fe₂O₃ 31.0 38.0 96.2 5 CuO Co₃O₄ 24.1 43.5 100 6 NaVO₃ MnO₂ 46.2 19.7 99.3 7 V₂O₅ MnO₂ 39.6 33.6 98.6 8 V₂O₅ CuO 37.6 17.0 98.1 9 MoO₃ Co₃O₄ 23.0 23.9 99.1 CE-1 CuO — 20.0 33.0 99.1 CE-2 NaVO₃ — 32.9 35.1 96.9

Examples 1 to 9 demonstrate that, in general, the binary catalyst system outperforms systems containing a single catalyst (Comparative Examples 1 and 2). Systems comprising a binary catalyst which failed to outperform the corresponding single catalyst system are regarded as not fully optimized in terms of the relative amounts of the components of the binary catalyst (See for example, Example 9 in which the conversion efficiencies for DBT and BT are relatively low in comparison to other Examples representing embodiments of the invention.

In each of Examples 1 to 9 the oxidized sulfur compounds may be separated from the reaction mixture (first oxidized mixture) using any of the techniques disclosed herein as being effective for that purpose. In one embodiment, the reaction mixture of Example 1 is filtered through a pad of silica gel to remove both the oxidized sulfur compounds and the supported exogenous binary catalyst which may be recovered therefrom.

Example 10

Saudi Crude 100 ml is first mixed with petroleum ether (PE) in a volume ratio of PE:Oil=2:1. The mixture is centrifuged at 2100 rpm for 10 min and then decanted to separate the solid asphaltenes and passed through an adsorption column filled with silica to remove heavy metals to provide a deasphalted oil fraction having a sulfur content of about 3.3 weight percent as determined using a Spectro Phoenix II XRF analyzer. 6.4 g of the deasphalted oil fraction and a supported exogenous binary catalyst 106 mg (same as that used in Example 3) are placed in a four-dram vial equipped with magnetic cross-like stirbars. The vials are placed in an aluminum heating block. The block with the vials is placed in a one-gallon autoclave equipped with a magnetic stirrer. The autoclave is maintained under an air pressure of 2000 pounds per square inch at a temperature of about 150° C. for a period of about 6 hours. The autoclave is then depressurized and the oxidized sample is washed with 2 ml 75 percent acetic acid and then with 2 ml of water. The oil fraction (6.1 g) contained 2.05 weight percent sulfur as analyzed by a Spectro Phoenix II XRF analyzer. This demonstrates that 37 percent of the sulfur containing components of the crude are converted into the corresponding oxidized derivatives (sulfones, sulfoxides) at about 95 percent yield of oil.

In one embodiment, the reaction mixture comprising the crude oil and the petroleum ether is filtered through a pad of silica gel to remove both the oxidized sulfur compounds and the supported exogenous binary catalyst which may be recovered therefrom. In general, the oxidized sulfur compounds may be separated from the crude oil containing reaction mixture (first oxidized mixture) using any of the techniques disclosed herein as being effective for that purpose.

The foregoing examples are merely illustrative, serving to illustrate only some of the features of the invention. The appended claims are intended to claim the invention as broadly as it has been conceived and the examples herein presented are illustrative of selected embodiments from a manifold of all possible embodiments. Accordingly, it is Applicants' intention that the appended claims are not to be limited by the choice of examples utilized to illustrate features of the present invention. As used in the claims, the word “comprises” and its grammatical variants logically also subtend and include phrases of varying and differing extent such as for example, but not limited thereto, “consisting essentially of” and “consisting of.” Where necessary, ranges have been supplied, those ranges are inclusive of all sub-ranges there between. It is to be expected that variations in these ranges will suggest themselves to a practitioner having ordinary skill in the art and where not already dedicated to the public, those variations should where possible be construed to be covered by the appended claims. It is also anticipated that advances in science and technology will make equivalents and substitutions possible that are not now contemplated by reason of the imprecision of language and these variations should also be construed where possible to be covered by the appended claims. 

1. A method for purifying a sulfur-containing fuel oil, the method comprising: (a) contacting the fuel oil with a supported exogenous binary catalyst and oxygen at a temperature in a range of from about 25° C. to about 150° C., and at a pressure in a range of from about 1 atmosphere to about 150 atmospheres to provide a first oxidized mixture; and (b) separating at least one oxidized sulfur compound from the first oxidized mixture to a provide a purified fuel oil.
 2. The method according to claim 1, wherein the sulfur-containing fuel oil comprises less than 5 weight percent sulfur.
 3. The method according to claim 1, wherein the sulfur-containing fuel oil comprises less than 3 weight percent sulfur.
 4. The method according to claim 1, wherein the supported exogenous binary catalyst comprises a metal oxide solid support selected from the group consisting of alumina, silica, magnesia, titania, ceria, and combinations of at least two of the foregoing.
 5. The method according to claim 4, wherein the metal oxide solid support is alumina.
 6. The method according to claim 1, wherein the exogenous binary catalyst comprises a first component selected from the group consisting of oxides and salts vanadium, manganese and copper, and combinations thereof, and a second component selected from the group consisting of oxides and salts of cerium, iron, titanium, manganese, cobalt, nickel, copper and combinations thereof.
 7. The method according to claim 6, wherein the first component comprises an oxide or a salt of vanadium.
 8. The method according to claim 6, wherein the first component comprises an oxide or a salt of manganese.
 9. The method according to claim 6, wherein the second component comprises an oxide or a salt of cobalt.
 10. The method according to claim 6, wherein the second component comprises an oxide or a salt of copper.
 11. The method according to claim 1, wherein the oxygen is provided as a mixture with an inert gas.
 12. The method according to claim 1, wherein the oxygen is provided as air.
 13. The method according to claim 1, wherein the sulfur-containing fuel oil is deasphalted prior to contacting the sulfur-containing fuel oil with the binary catalyst, hydrogen peroxide and the water-soluble acid by contacting the sulfur-containing fuel with an inert diluent.
 14. The method according to claim 13, wherein the inert diluent is selected from the group consisting of liquid saturated hydrocarbons, liquid cyclic hydrocarbons, and mixtures of at least two of the foregoing inert diluents.
 15. The method according to claim 13, wherein the inert diluent is selected from the group consisting of propane, butane, petroleum ether, cyclohexane, decalin and mixtures thereof.
 16. The method according to claim 1, wherein the separating is carried out using solid-liquid extraction.
 17. The method according to claim 1, wherein the separating is carried out using liquid-liquid extraction.
 18. The method according to claim 1, wherein the sulfur-containing fuel oil comprises benzothiophene, dibenzothiophene, alkyl substituted benzothiophenes, and alkyl substituted dibenzothiophenes.
 19. The method according to claim 1, further comprising a step of recovering the binary catalyst.
 20. A method for purifying a sulfur-containing fuel oil, the method comprising: (a) contacting the sulfur-containing fuel oil with a hydrocarbon diluent, an alumina supported exogenous binary catalyst, and oxygen at a temperature in a range of from about 50° C. to about 120° C., and at a pressure in a range of from about 1 atmosphere to about 150 atmospheres to provide a first oxidized mixture; (b) separating at least one oxidized sulfur compound from the first oxidized mixture; and (c) recovering the diluent to provide a purified fuel oil.
 21. The method according to claim 20, wherein the exogenous binary catalyst comprises a first component selected from the group consisting of oxides and salts vanadium, manganese and copper, and combinations thereof, and a second component selected from the group consisting of oxides and salts of cerium, iron, titanium, manganese, cobalt, nickel, copper and combinations thereof.
 22. The method according to claim 20, further comprising a step of recovering the binary catalyst.
 23. A method for purifying a sulfur-containing fuel oil, the method comprising: (a) contacting a sulfur-containing fuel oil comprising benzothiophene, dibenzothiophene, alkyl substituted benzothiophenes, and alkyl substituted dibenzothiophenes with petroleum-ether, a supported exogenous binary catalyst, and oxygen at a temperature in a range of from about 50° C. to about 120° C., and at a pressure in a range of from about 1 atmosphere to about 150 atmospheres to provide a first oxidized mixture comprising sulfoxides and sulfones of benzothiophene, dibenzothiophene, alkyl substituted benzothiophenes, and alkyl substituted dibenzothiophenes; (b) separating at least one oxidized sulfur compound from the first oxidized mixture; and (c) recovering petroleum-ether to provide a purified fuel oil.
 24. The method according to claim 23, wherein the exogenous binary catalyst comprises a first component selected from the group consisting of oxides and salts vanadium, manganese and copper, and combinations thereof, and a second component selected from the group consisting of oxides and salts of cerium, iron, titanium, manganese, cobalt, nickel, copper and combinations thereof.
 25. The method according to claim 23, further comprising a step of recovering the binary catalyst. 